The present invention relates to an optical information recording and reproducing apparatus for recording, reproducing and erasing required information optically by an irradiating light beam emanated from a light source, such as a semiconductor laser onto an information recording medium, and particularly to an apparatus for recording and reproducing optical information in which light output from the light source is most suitably adjusted in accordance with each operation mode of recording, reproducing and erasing and a focussing control (focus adjustment) and/or a tracking control (positional adjustment of light beam for tracking a track of the information recording medium) is most suitably performed.
In optical information recording and reproducing apparatus for recording, reproducing and erasing required information on the information recording apparatus by scanning a light beam from a light source on the information recording medium (referred to as a disc) as a spot light, the light quantity control of the light source is inevitable in order to perform a stable operation at each mode of recording, reproducing and erasing. Particularly, in an optical disc utilizing a disc-shaped information recording medium, there has been proposed a system for automatically adjusting light intensity in accordance with a position of the light beam to a radial direction of the disc (referred to as an automatic power control), since a relative rotation speed to the light beam irradiated on to the information recording medium is different at an inner periphery portion and an outer periphery portion of the disc which rotates with a certain rotational speed (refer to, for example, Japanese Patent Laid-open No. 59-146,457).
In order to perform optimum focus adjustment and positional control of the light beam to the track, there have been proposed a focussing control and a tracking control in which the light reflected from or transmitted through the optical disc (hereinafter referred to as a return light) is detected by a light detector which is divided into four equal portions, to obtain an error signal, thereby shifting lens position or optical pick-up through a closed servo loop. In such a control of the light beam due to the closed servo loop, however, contamination or deterioration of optical components or parts such as, for example, lens or mirror affects a level of the detected return light so that the level of the error signal is changed resulting in a change in a total gain of a control loop and thus a followability of the servo control becomes bad or an oscillatory phenomena arises resulting in a cause of unstable operation. In an optical disc having a specidal construction such as a groove structure or pit structure in which a groove shaped track is previously provided on the optical disc, the same phenomena causes a change in light quantity of the return light. Then, it has been proposed an automatic gain control system in which the return light is detected and a circuitry gain in the focussing loop is changed in accordance with the total detected light quantity, thereby absorbing a change in total gain caused by the contamination and deterioration of optical components (for example, refer to Japanese Patent Laid-open No. 56-148,745).
As described above, according to the prior art, the control loop for performing the focussing control or the tracking control consists of a closed servo loop in which the return light from the optical disc is detected, the gain of the control loop is adjusted on the basis of the detected signal level, and the level of the lens drive signal or the optical pick-up drive signal is adjusted thereby performing the focussing control or the tracking control. In this case, for example, when the light beam is shifted from the inner periphery portion to the outer periphery portion of the optical disc on the basis of the external instruction, the automatic power control device receives the positional signal corresponding to the radial position of the disc thereby increasing the light output level of the light source with good responsibility, but the servo loop for the focussing and tracking control adjusts the total gain thereof only after the return light is detected in accordance with the light quantity increased by the automatic power control device thereby performing the focussing and tracking control. At the time the light beam has been shifted in the radial direction of the disc, the gain of the servo loop is not yet adjusted completely so that it is difficult and unstable to perform correct focussing and tracking to the desired or aimed track and under certain circumstances it is impossible to perform tracking due to the oscillation.